This invention relates to fluid bearings and particularly to such bearings between a magnetic tape and guide drums for helical scanning of said tape.
In U.S. Pat. No. 4,184,183 (Dolby), there is disclosed an arrangement in which a magnetic tape may be helically scanned by two transducing heads concurrently, the heads being mounted in axially-spaced and circumferentially-spaced relation on a rotating scanning drum. The rotating drum is mounted coaxially with respect to a stationary drum of approximately the same diameter and is axially spaced to define a small axial gap between the drums. The tape is wrapped helically around the drums and crosses the gap. A pressurized air film is generated beneath the tape by the rotating drum, and although vented to some degree by the gap, extends nevertheless beneath the tape as it crosses the stationary drum as well. A first of a pair of transducing heads is mounted on the rotating drum at the edge of the gap, and the second head is mounted on a solid (edgeless) portion of the rotating drum, spaced axially and circumferentially from the first head.
It has been found in practice that the venting of the air bearing film by the gap between the drums, together with lateral leakage from beneath the tape edges, causes a substantial decreasing gradient of the bearing film thickness from a maximum thickness over the rotating drum to a much lower thickness over the stationary drum, which decrease in turn appears to cause a variation in both recorded and playback signal strength, as observed by oscilloscope in reproduce mode from a low value at the beginning of the scan of either head, to a higher value which remains substantially uniform over the middle and end portions of the head scan path.
These observations are consistent with capacity probe measurements of the film thickness at various points, and suggests that the gap causes a gradual drop in the film thickness from a maximum dimension just downstream (in the direction of gas flow and drum rotation) from the gas entrance region of the bearing, at the point of tape tangency with the rotating drum.
Such variation of film thickness is of course not desired, for it causes a corresponding variation in the general level of the signal strength, and consequently a variation in the quality of recording and replay.
It is therefore an object of the present invention to provide means for controlling the thickness of a pressurized bearing film between a tape and a pair of coaxially-mounted guides having differential rotation with respect to one another.